FIG. 1 is a structural diagram illustrating a network structure of an Evolved Universal Mobile Telecommunication System (E-UMTS) acting as a fourth-generation mobile communication system. The E-UMTS system is developed from a conventional UMTS system, and conducts intensive research into a basic standardization process in the current 3GPP. The E-UMTS system may also be called a Long Term Evolution (LTE) system.
The E-UMTS network is classified into an Evolved UTRAN (E-UTRAN) and an Evolved Packet Core (EPC). The E-UTRAN includes a user equipment (UE), an eNode-B, an Access Gateway (AG) which is located at the end of a network simultaneously while being connected to an external network, and a Mobility Management Entity (MME)/User Plane Entity (UPE). The AG may be divided into a first AG part for taking charge of user traffic and a second AG part for taking charge of control traffic.
In this case, a new interface may be located between the first AG part for processing the new user traffic and the second AG part for processing the control traffic, such that the first AG part may communicate with the second AG part. A single eNode-B may include at least one cell. An interface for transmitting either the user traffic or the control traffic may be located between the eNode-Bs. The EPC may include an AG or a node for user registration of other UEs, etc. An interface for distinguishing the E-UTRAN from the EPC may also be used. Several nodes are located between the eNode-B and the AG via the S1 interface. In this case, the several nodes are interconnected (i.e., Many to Many Connection). Several eNode-Bs are interconnected via the X2 interface, and a meshed network which always has the X2 interface is located between the eNode-Bs.
Radio protocol layers between the user equipment (UE) and the network are classified into a first layer (L1), a second layer (L2), and a third layer (L3) of an Open System Interconnection (OSI) reference model well known to a communication system. A physical layer contained in the L2 layer provides an information transfer service using a physical channel. A radio resource control (RRC) layer located in a third layer controls radio resources between the UE and the network. For this operation, the RRC message is exchanged between the UE and the network in the RRC layer. The RRC layer is located at the eNode-B in the E-UTRAN network.
FIG. 2 is a structural diagram illustrating a radio protocol layer structure between the UE and the E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) on the basis of the 3GPP radio access network standard acting as the third-generation mobile communication standardization organization. The radio protocol layer structure of FIG. 2 horizontally includes a physical layer, a data link layer, and a network layer. The radio protocol layer structure of FIG. 2 vertically includes a user plane for transmitting data information and a control plane for transmitting a control signal (i.e., signaling information). The radio protocol layers of FIG. 2 are classified into a first layer (L1), a second layer (L2), and a third layer (L3) of an Open System Interconnection (OSI) reference model well known to a communication system.
The control plane and the user plane in the radio protocol layer structure of FIG. 2 will hereinafter be described. The first layer (L1) is a physical layer. The physical layer provides an upper layer with an information transfer service over a physical channel. The physical layer is connected to an upper medium access control (MAC) layer over a transport channel. Data is communicated between the MAC layer and the physical layer via the transport channel. Data is communicated between different physical layers over a physical channel. Namely, Data is communicated between a first physical layer of a transmission end and a second physical layer of a reception end.
The medium access control (MAC) layer of the second layer transmits services to a radio link control (RLC) layer acting as an upper layer over a logical channel. The RLC layer supports transmission of reliable data. A function of the RLC layer may be implemented with any function block contained in the MAC. In this case, it should be noted that there is no RLC layer. PDCP layer of the second layer (L2) performs a header compression function for reducing the IP packet header size including relatively large- and unnecessary-control information, such that it can effectively transmit IP packet (such as IPv4 or IPv6) within a small-bandwidth RF interval. The PDCP layer of the E-UTRAN is located at the AG.
A Radio Resource Control (RRC) layer located at the uppermost of the third layer is defined in only the control plane. The RRC layer is associated with configuration, re-configuration, and release of a radio bearer (RB), such that it controls a logical channel, a transport channel, and a physical channel. In this case, the radio bearer (RB) is indicative of a service provided from a second layer to implement data communication between the UE and the E-UTRAN.
The unit of data transmitted to each layer of the radio protocol layer structure is called different names. This data unit is called a service data unit (SDU). A basic unit for allowing a protocol to transmit data to another layer is called a protocol data unit (PDU). Data which moves between layers of a radio access protocol structure or between radio access protocol structures is indicative of a predetermined data block such as the above-mentioned SDU or PDU.
The RLC layer will hereinafter be described in detail. A basic function of the RLC layer guarantees a Quality of Service (QoS) of each RB and transmits data for the guaranteed QoS. The RB service allows the second layer (L2) of the radio protocol to be provided to an upper part, such that a total of the second layer (L2) affects a QoS of the RB. Specifically, it should be noted that the RB QoS is more affected by the RLC of the second layer (L2). The RLC has an independent RLC entity for each RB to guarantee a unique QoS of the RB, and provides three RLC modes to support a variety of quality of services (QoSs). The three RLC modes are a transparent mode (TM) RLC mode, an unacknowledged mode (UM) RLC mode, and an acknowledged mode (AM) RLC mode. The above-mentioned RLC modes support different QoSs, have different operation methods, and have different detailed functions. Next, the above-mentioned RLC operation modes will hereinafter be described in detail.
When the PDU (hereinafter referred to as RLC PDU) is configured in the RLC layer, the TM RLC attaches no overhead to the SDU (hereinafter referred to as RLC SDU) of the RLC layer received from an upper layer. In other words, the RLC transparently passes through the SDU, such that this RLC is called a TM RLC. Due to this feature, the following operations are performed in the user plane and the control plane. The user plane has a short data processing time in the RLC, and the control plane has no overhead in the RLC, such that an uplink conducts transmission of the RRC message from an unspecified UE and a downlink conducts transmission of the RRC message broadcast to all UEs contained in a cell.
If overhead is added to the RLC differently from the transparent mode (TM) mode, this mode is called a non-transparent mode. The non-transparent mode has the UM RLC mode having an acknowledgment (ACK) signal of Tx data and the AM RLC mode having no ACK signal of the Tx data. The UM RLC attaches a PDU header including a sequence number to each PDU, and transmits the attached result, such that a reception end can recognize which one of PDUs has been lost during a transmission time. Due to this function, in the user plane, the UM RLC mainly transmits broadcast/multicast data or transmits real-time packet data (e.g., VoIP or streaming) of a packet service (PS) domain. In the control plane, the UM RLC transmits a specific RRC message, which does not require the ACK signal, from among several RRC messages transmitted to a specific UE or a specific UE group in a cell.
The AM RLC from among the non-transparent modes attaches the PDU header including a sequence number to the PDU in the same manner as in the UM RLC. Differently from the UM RLC, in the case of the AM RLC, the reception end transmits the acknowledgment (ACK) signal to the PDU received from the transmission end. The reason why the reception end transmits the acknowledgment (ACK) signal in the AM RLC is that the transmission end requests retransmission of the PDU which has not been received in the reception end. This retransmission function is the highest feature of the AM RLC. The AM RLC aims to guarantee transmission of error-free data for the above retransmission. Due to this purpose, in the user plane, the AM RLC mainly conducts transmission of non-realtime packet data such as TCP/IP of a PS domain. In the control plane, the AM RLC transmits a specific RRC message, which requires the ACK signal, from among several RRC messages transmitted to a specific UE in a cell.
From the viewpoint of directivity, the TM RLC or the UM RLC is used to implement unidirectional communication. The AM RLC receives a feedback message from a reception end, such that it is used to implement bidirectional communication. This bidirectional communication has been widely used to implement a point-to-point communication, such that the AM RLC uses only a dedicated logical channel. There is a difference in structure between the TM RLC and the UM RLC. In the case of the TM or UM RLC, a single RLC entity has a single Tx or Rx structure, but the AM RLC includes both the Tx structure and the Rx structure in a single RLC entity.
The AM RLC has a retransmission buffer other than a Tx/Rx buffer in order to implement retransmission management. Besides, the AM RLC performs a variety of techniques to control the signal flow. For example, a Tx/Rx window, a polling message, a status report message, a status PDU, and a piggyback may be used. The polling message allows the transmission end to request status information from the reception end of the RLC entity. The status report message allows the reception end to report its buffer status to the transmission end of the peer RLC entity. The status PDU is used to carry status information. The piggyback is used to increase the efficiency of data transmission, such that the status information PDU is inserted into the data PDU. In order to support the above-mentioned functions, the AM RLC requires a variety of protocol parameters, status variables, and a timer. PDU for reporting the above status information, the status information PDU, and PDUs for controlling transmission of data in the AM RLC are called control PDUs. PDUs for transmitting user data are called data PDUs.
FIG. 3 is a flow chart illustrating an AM RLC PDU retransmission process of the AM RLC entity of a reception end according to the conventional art. Referring to FIG. 3, the AM RLC entity (hereinafter referred to as a transmission end) of the transmission end transmits PDU1, PDU2, and PDU3 of the AM RLC layer such that data is transmitted to the AM RLC entity (hereinafter referred to as a reception end) of the reception end at step S300. However, only PDU1 and PDU3 are received in the reception end at step S310.
The reception end is able to transmit a specific status report message indicating whether data has been received at intervals of a predetermined time to the transmission end. This transmission may be called a periodic status report transmission scheme. In other words, the reception end transmits the status PDU including the periodic status report to the transmission end at a specific time at which the timer driven at intervals of a predetermined period has expired. Then, the reception end resets the timer, and operates the timer. In FIG. 3, if a periodic status report timer of the reception end has expired at step S320, the reception end transmits status information ‘PDU2’ indicating non-received PDU2, and informs the transmission end of the status information at step S330. The transmission end retransmits ‘PDU2’ on the basis of the status information PDU2 received from the reception end at step S340. However, since the periodic status report timer has expired before the retransmitted PDU2 arrives at the reception end, the reception end transmits the status information PDU3 indicating non-received PDU2 at step S345. The transmission end which has received the status information PDU3 retransmits ‘PDU2’ at step S355.
Thereafter, the reception end receives the retransmitted PDU2 of the above step S340 without any error at step S350, and a Tx/Rx process of the AM RLC PDU is completed. However, if a new periodic status report time has expired at the reception end, the status information PDU4 is transmitted to the transmission end at step S360.
The above-mentioned status information PDU report at the reception end is conducted at intervals of a single fixed period without considering data retransmission status of the transmission end, and may encounter unnecessary consumption of radio resources without reflecting a data Tx situation between the transmission end and the reception end.